This invention relates to an apparatus for illuminating an area of a particular range and, more particularly, to an apparatus for illuminating a photomask or reticle having an integrated circuit pattern during a step for transferring the pattern onto a semiconductor wafer.
Manufacture of semiconductor devices such as integrated circuits involves pattern transfer steps relative to the same semiconductor wafer for superposing, on the semiconductor wafer in a predetermined sequence, the patterns formed on various photomasks or reticles. The pattern transfer step is carried out by illuminating the photomask or reticle (which hereinafter will be referred to simply as "mask") held in contact with or in close proximity to the wafer having on its surface a sensitive layer or by illuminating the pattern which is located at a position optically conjugate with the sensitive layer on the wafer with respect to a refraction type or reflection type projection lens system.
Since the pattern transfer is effected by the illumination of the pattern, the image quality of the pattern transferred onto the wafer is affected to a great extent by the performances of the illumination apparatus for illuminating the mask. In view of this, many proposals have been made with reference to the illumination apparatus, such as disclosed in U.S. Pat. No. 3,795,446 issued Mar. 5, 1974, U.S. Pat. No. 3,988,066 issued Oct. 26, 1976 and U.S. Pat. No. 4,375,315 issued Mar. 1, 1983 and Japanese Laid-Open patent application No. 81813/1981 laid open July 4, 1981.
Essential considerations for the illumination apparatus to be used in transferring the integrated circuit pattern onto the semiconductor wafer are as follows:
(1) Removal of unevenness in the illuminance on the mask surface;
(2) Improvement of the illuminance; and
(3) Achieving an appropriate angle of incidence of the light beam incident on the mask surface.
In order to remove the unevenness in the illuminance (Item 1), it has been proposed to use a multi-beam generating member. The multi-beam generating member comprises a fly's eye lens (compound eye lens), an optical element formed by a bundle of self-converging fibers or a bundle of ordinary fibers, or a combination of radially disposed prisms. By such multi-beam generating member, a number of descrete light beams (fluxes) are generated, which beams are collimated by a collimator so that they are integrated on the pattern surface. By doing so, the illuminance unevenness is maintained within a range of .+-.2-3%. The improvement of illuminance (Item 2) has been promoted by increasing the light-collecting efficiency or by increasing the luminance of the light source. The appropiate angle of incidence of the light beam (Item 3) has been attained by selecting the diameter of the multi-beam generating member and the focal length of the collimation lens.
In addition to the above-described requirements, attention has recently been paid to uniformity of intensity distribution of the light beam impinging on the mask surface. The inventor of the present invention has already proposed in U.S. patent application Ser. No. 508,333 filed June 27, 1983, corresponding to Japanese patent application No. 115846/1982 filed July 2, 1982, an illumination apparatus wherein a plurality of multi-beam generating members are disposed in the path of illumination downstream of an elliptical reflecting mirror in order to compensate for an uneven intensity distribution which is produced in the image of the light source formed by the elliptical reflecting mirror, to thereby assure uniform intesity distribution of the light beam impinging on the pattern surface. Since, however, the multi-beam generating member causes approximately 20-40% loss in the light quantity, use of plural multi-beam generating members inevitably decreases the collecting efficiency.